<!DOCTYPE html>
<html prefix="og: http://ogp.me/ns#">
<head>
    <meta charset='utf-8'>
    <title>Fresco | 数据源和数据订阅者</title>
    <link rel='stylesheet' href='/static/site.css' type='text/css'/>
    <link rel="stylesheet" href='/static/pygments.css' type='text/css'/>
    <link rel='shortcut icon' href='/static/favicon.png'>
    <meta name='viewport' content='width=480'>
    <meta property="og:title" content="数据源和数据订阅者" />
    <meta property="og:site_name" content="fresco">
    <meta property='og:description' content='Fresco is an image management library.'>
    <meta property='og:image' content='http://facebook.github.io/fresco/static/fresco-og-image.png'>
    <meta property='og:url' content="http://facebook.github.io/fresco/docs/datasources-datasubscribers.html">
    
    <meta property='og:type' content='website'>
    
    <script type="text/javascript" src="//use.typekit.net/vqa1hcx.js"></script>
    <script type="text/javascript">{'try{Typekit.load();}catch(e){}'}</script>
    <!-- Google Analytics -->
    <script>
      (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
      (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
      m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
      })(window,document,'script','//www.google-analytics.com/analytics.js','ga');

      ga('create', 'UA-43024238-8', 'auto');
      ga('send', 'pageview');

    </script>
</head>
<body>

<header class='topbar'><nav class='width'>
<a href='/'>
  <img src="/static/fresco-logo.png" class="logo">
</a>
    <ul>
      <li><a href="/docs/index.html#_"
          
          class="active" >
          文档
      </a></li>
      <li><a href="/"
          >
          关于
      </a></li>
      <li><a href="/support.html"
          >
          支持
      </a></li>
      <li><a href="/javadoc">
          API
      </a></li>
      <li><a href="http://github.com/facebook/fresco">GitHub</a>
      <li><a href="/about.html"
          >
      关于本文档</a></li>
    </ul>
</nav></header>


<section class='content'><div class='width'>

<nav class='toc'>
  
    <section>
      <h3>配置和使用</h3>
      <ul>
        
          <li>
            <a href="/docs/download-fresco.html#_">
              下载
            </a>
            
          </li>
        
          <li>
            <a href="/docs/index.html#_">
              配置和开始使用
            </a>
            
          </li>
        
          <li>
            <a href="/docs/concepts.html#_">
              关键概念
            </a>
            
          </li>
        
          <li>
            <a href="/docs/supported-uris.html#_">
              支持的 URIs
            </a>
            
          </li>
        
      </ul>
    </section>
  
    <section>
      <h3>Drawee 指南</h3>
      <ul>
        
          <li>
            <a href="/docs/using-drawees-xml.html#_">
              在XML中使用 Drawees
            </a>
            
          </li>
        
          <li>
            <a href="/docs/using-drawees-code.html#_">
              在Java代码中使用 Drawees
            </a>
            
          </li>
        
          <li>
            <a href="/docs/drawee-components.html#_">
              Drawee的各种效果配置
            </a>
            
          </li>
        
          <li>
            <a href="/docs/scaling.html#_">
              缩放
            </a>
            
          </li>
        
          <li>
            <a href="/docs/rounded-corners-and-circles.html#_">
              圆角/圆圈
            </a>
            
          </li>
        
          <li>
            <a href="/docs/using-controllerbuilder.html#_">
              使用ControllerBuilder
            </a>
            
          </li>
        
          <li>
            <a href="/docs/progressive-jpegs.html#_">
              渐进式JPEG图
            </a>
            
          </li>
        
          <li>
            <a href="/docs/animations.html#_">
              动画图(gif)
            </a>
            
          </li>
        
          <li>
            <a href="/docs/requesting-multiple-images.html#_">
              多图请求及图片复用
            </a>
            
          </li>
        
          <li>
            <a href="/docs/listening-download-events.html#_">
              监听下载事件
            </a>
            
          </li>
        
          <li>
            <a href="/docs/resizing-rotating.html#_">
              缩放和旋转
            </a>
            
          </li>
        
          <li>
            <a href="/docs/modifying-image.html#_">
              图片的修改
            </a>
            
          </li>
        
          <li>
            <a href="/docs/image-requests.html#_">
              图片请求
            </a>
            
          </li>
        
          <li>
            <a href="/docs/writing-custom-views.html#_">
              自定义View
            </a>
            
          </li>
        
          <li>
            <a href="/docs/gotchas.html#_">
              一些陷阱
            </a>
            
          </li>
        
      </ul>
    </section>
  
    <section>
      <h3>Image Pipeline 指南</h3>
      <ul>
        
          <li>
            <a href="/docs/intro-image-pipeline.html#_">
              什么是Image Pipeline
            </a>
            
          </li>
        
          <li>
            <a href="/docs/configure-image-pipeline.html#_">
              配置Image Pipeline
            </a>
            
          </li>
        
          <li>
            <a href="/docs/caching.html#_">
              缓存的行为控制
            </a>
            
          </li>
        
          <li>
            <a href="/docs/using-image-pipeline.html#_">
              直接使用Image Pipeline
            </a>
            
          </li>
        
          <li>
            <a href="/docs/datasources-datasubscribers.html#_" class="active">
              数据源/订阅数据源
            </a>
            
          </li>
        
          <li>
            <a href="/docs/closeable-references.html#_">
              可关闭的引用
            </a>
            
          </li>
        
      </ul>
    </section>
  
    <section>
      <h3>第三方类库</h3>
      <ul>
        
          <li>
            <a href="/docs/using-other-network-layers.html#_">
              自定义网络加载
            </a>
            
          </li>
        
          <li>
            <a href="/docs/using-other-image-loaders.html#_">
              使用其他的Image Loader
            </a>
            
          </li>
        
      </ul>
    </section>
  
</nav>


<article class='withtoc'>
    <h1>
      数据源和数据订阅者
      <a class="edit-page-link" href="https://github.com/liaohuqiu/fresco-docs-cn/blob/docs/docs/02-datasources-datasubscribers.md" target="_blank">编辑和纠错</a>
    </h1>
    <p></p>

    <p><a href="../javadoc/reference/com/facebook/datasource/DataSource.html">数据源</a> 和 <a href="http://developer.android.com/reference/java/util/concurrent/Future.html">Future</a>, 有些相似，都是异步计算的结果。</p>

<p>不同点在于，数据源对于一个调用会返回一系列结果，Future只返回一个。</p>

<p>提交一个Image request之后，Image
pipeline返回一个数据源。从中获取数据需要使用<a href="../javadoc/reference/com/facebook/datasource/DataSubscriber.html">数据订阅者(DataSubscriber)</a>.</p>

<h3>当你仅仅需要Bitmap</h3>

<p>如果你请求Image pipeline仅仅是为了获取一个 <a href="http://developer.android.com/reference/android/graphics/Bitmap.html">Bitmap</a>, 对象。你可以利用简单易用的<a href="../javadoc/reference/com/facebook/imagepipeline/datasource/BaseBitmapDataSubscriber">BaseBitmapDataSubscriber</a>:</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">dataSource</span><span class="o">.</span><span class="na">subscribe</span><span class="o">(</span><span class="k">new</span> <span class="nf">BaseBitmapDataSubscriber</span><span class="o">()</span> <span class="o">{</span>
    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">onNewResultImpl</span><span class="o">(</span><span class="nd">@Nullable</span> <span class="n">Bitmap</span> <span class="n">bitmap</span><span class="o">)</span> <span class="o">{</span>
       <span class="c1">// You can use the bitmap in only limited ways</span>
      <span class="c1">// No need to do any cleanup.</span>
    <span class="o">}</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">onFailureImpl</span><span class="o">(</span><span class="n">DataSource</span> <span class="n">dataSource</span><span class="o">)</span> <span class="o">{</span>
      <span class="c1">// No cleanup required here.</span>
    <span class="o">}</span>
  <span class="o">});</span>
</code></pre></div>
<p>看起来很简单，对吧。下面是一些小警告:</p>

<p>千万 <strong>不要</strong> 把bitmap复制给<code>onNewResultImpl</code>函数范围之外的任何变量。订阅者执行完操作之后，image
pipeline
会回收这个bitmap，释放内存。在这个函数范围内再次使用这个Bitmap对象进行绘制将会导致<code>IllegalStateException</code>。</p>

<h3>通用的解决方案</h3>

<p>如果你就是想维持对这个Bitmap对象的引用，你不能维持纯Bitmap对象的引用，可以利用[可关闭的引用(closeable
references)[closeable-references.html) 和 <a href="../javadoc/reference/com/facebook/datasource/BaseDataSubscriber.html">BaseDataSubscriber</a>:</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">DataSubscriber</span> <span class="n">dataSubscriber</span> <span class="o">=</span>
    <span class="k">new</span> <span class="n">BaseDataSubscriber</span><span class="o">&lt;</span><span class="n">CloseableReference</span><span class="o">&lt;</span><span class="n">CloseableImage</span><span class="o">&gt;&gt;()</span> <span class="o">{</span>
  <span class="nd">@Override</span>
  <span class="kd">public</span> <span class="kt">void</span> <span class="nf">onNewResultImpl</span><span class="o">(</span>
      <span class="n">DataSource</span><span class="o">&lt;</span><span class="n">CloseableReference</span><span class="o">&lt;</span><span class="n">CloseableImage</span><span class="o">&gt;&gt;</span> <span class="n">dataSource</span><span class="o">)</span> <span class="o">{</span>

    <span class="k">if</span> <span class="o">(!</span><span class="n">dataSource</span><span class="o">.</span><span class="na">isFinished</span><span class="o">())</span> <span class="o">{</span>
      <span class="n">FLog</span><span class="o">.</span><span class="na">v</span><span class="o">(</span><span class="s">&quot;Not yet finished - this is just another progressive scan.&quot;</span><span class="o">);</span>
    <span class="o">}</span>  

    <span class="n">CloseableReference</span><span class="o">&lt;</span><span class="n">CloseableImage</span><span class="o">&gt;</span> <span class="n">imageReference</span> <span class="o">=</span> <span class="n">dataSource</span><span class="o">.</span><span class="na">getResult</span><span class="o">();</span>
    <span class="k">if</span> <span class="o">(</span><span class="n">imageReference</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
      <span class="k">try</span> <span class="o">{</span>
        <span class="n">CloseableImage</span> <span class="n">image</span> <span class="o">=</span> <span class="n">imageReference</span><span class="o">.</span><span class="na">get</span><span class="o">();</span>
        <span class="c1">// do something with the image</span>
      <span class="o">}</span> <span class="k">finally</span> <span class="o">{</span>
        <span class="n">imageReference</span><span class="o">.</span><span class="na">close</span><span class="o">();</span>
      <span class="o">}</span>
    <span class="o">}</span>
  <span class="o">}</span>
  <span class="nd">@Override</span>
  <span class="kd">public</span> <span class="kt">void</span> <span class="nf">onFailureImpl</span><span class="o">(</span><span class="n">DataSource</span> <span class="n">dataSource</span><span class="o">)</span> <span class="o">{</span>
    <span class="n">Throwable</span> <span class="n">throwable</span> <span class="o">=</span> <span class="n">dataSource</span><span class="o">.</span><span class="na">getFailureCause</span><span class="o">();</span>
    <span class="c1">// handle failure</span>
  <span class="o">}</span>
<span class="o">};</span>

<span class="n">dataSource</span><span class="o">.</span><span class="na">subscribe</span><span class="o">(</span><span class="n">dataSubscriber</span><span class="o">,</span> <span class="n">executor</span><span class="o">);</span>
</code></pre></div>
<p>这样，只要遵守<a href="closeable-references.html">可关闭的引用使用规则</a>，你就可以把这个<code>CloseableReference</code>复制给其他变量了。</p>


    <div class="docs-prevnext">
      
        <a href="/docs/using-image-pipeline.html#_">&larr; 上一页</a>
      
      
        <a class="right" href="/docs/closeable-references.html#_">下一页 &rarr;</a>
      
    </div>

    <a id="_"></a>
</article>

</div></section>
<footer><div class='width'>
    &copy; Copyright 2015, Facebook Inc.
</div></footer>

<script src="/static/linkify.js"></script>
<!-- Google Analytics -->
</body>
</html>

